The invention relates to an optical fiber amplifier and its amplification method, more particularly to an automatic gain control of an optical fiber amplifier using a rare earth doped optical fiber in the optical communication system.
FIG. 19 is a conventional optical fiber amplifier shown in the preliminary report C-281, "light soliton amplification and propagation using Er doped optical fiber", of the national meeting in autumn, the Institute of Electronics, Information and Communication Engineers, 1989, pp4-221. In FIG. 19, 1 is an optical fiber doped with rare earth. 2 is an excitation light source. 3 is a excitation light source. 4 and 5 are optical couplers. 6a is an input terminal of the optical signal. 6b is an output terminal of the optical signal.
An operation of the conventional optical fiber amplifier shown in FIG. 19 is explained hereinafter. In FIG. 19, the optical fiber doped with rare earth 1 is fabricated by doping the rare earth element erbium (Er) into a single mode fiber having the length of around several meters to several tens of meters. The optical fiber doped with rare earth 1 is connected to the optical coupler 5. The excitation light sources 2 and 3 comprise, for example, a semiconductor laser having a wavelength of 1.48 .mu.m.
The output lights from the excitation light sources 2 and 3 are synthesized to one light beam and inputted to the optical fiber doped with rare earth 1 through the optical coupler 5. When the excitation light sources 2 and 3 are inputted to the optical fiber doped with rare earth 1 through the optical coupler 5, the optical fiber doped with rare earth 1 changes to the state of inverted population. In the inverted population state, a optical signal having a wavelength of 1.53 .mu.m or 1.55 .mu.m inputted from the input terminal 1 is amplified by the stimulated emission effect and outputted to the output terminal 6b.
FIG. 20 shows the relation between excitation light source input power and optical signal output power. FIG. 21 shows the relation between optical signal input power and optical signal output power.
The conventional optical fiber amplifier shown in FIG. 19 has no control system to the output optical signal. Accordingly the output optical signal level easily varies according to the variations of the input optical signal, the output light of the excitation light source, the wavelength, the optical fiber doped with rare earth and the loss of the loptical coupler. If the optical communication system is constructed using the above conventional optical fiber amplifier, since the dynamic range required for each apparatus in the system is enlarged, an operational stability of the system is difficult to attain and the construction cost increases.
It is a primary object of the present invention to provide an optical fiber amplifier and a fiber amplification method, where the output optical signal level is maintained stable if the input optical signal, the output light of the excitation light source, the wavelength, the loss of the optical fiber doped with rare earth and the optical coupler varies, and stable operation is obtained if the optical fiber system has a plurality of excitation light sources.